1. Field of the Invention
This invention relates to electrical switching apparatus such as power circuit breakers, network protectors, transfer switches, and other switches used in electrical power circuits carrying large currents. More particularly, it relates to such switches having a cam assembly for applying a spring force to close the switch.
2. Background Information
Electrical switching apparatus for opening and closing electric power circuits typically utilize an energy storage device in the form of one or more large springs to close the contacts of the device into the large currents which can be drawn in such circuits. Such electrical switching apparatus includes power circuit breakers and network protectors, which provide protection, and electric switches, which are used to energize and de-energize parts of the circuit or to transfer between alternative power sources. These devices also include an opening spring or springs which rapidly separate the contacts to interrupt current flowing in the power circuit. As indicated, either or both of the closing spring and the opening spring can be a single spring or multiple springs and should be considered as either, even though the singular is hereafter used for convenience.
In a common arrangement, force stored in the close spring is transmitted to the moveable contact carrier of the switch through a drive cam and drive coupling arrangement. The drive cam is rotated by the close spring. This cam has a peripheral cam profile which varies in radius. The drive coupling includes a drive roller on a free end of a drive link connected through a bell crank to the pole shaft of the switch. A latch assembly latches the drive roller against the drive cam profile so that rotation of the drive cam by the closing spring results in rotation of the pole shaft, which being connected to the moveable contact carriers in each pole, results in closure of the switch contacts. When the latch is released, the drive roller floats allowing the open spring to rotate the movable contact carriers to open the switch contacts.
It is common to require that the switch have the capability of an immediate reclose following a trip or an intentional opening of the contacts. This makes it necessary to recharge the closing spring after it has been used to close the switch contacts. In order to allow the mechanism to reset, a reset cavity is provided in the drive cam. This results in a discontinuity in the profile of the drive cam and the formation of a tip at the end of the close section of the cam profile where the radius rapidly drops off to form the reset cavity. The latch assembly typically has a reset spring which pulls the driver roller into this cavity.
When the contacts are closed and the closing spring is recharged, the drive roller engages the drive cam profile adjacent the tip, poised for a rapid reset when the latch is released. This arrangement has worked very well in many models of power switches. Recently, there have been efforts to increase the ratings of these power switches. The withstand currents at these higher ratings, which are the short circuit currents that the switch such as a circuit breaker must tolerate to provide time for downstream circuit breakers to respond, develop forces that cause elastic deflection in the drive coupling. This leads to the drive roller creeping off of the close section of the cam profile and dropping into the reset cavity. The resulting premature trip has adverse consequences. The coordination between switches in a distribution system is upset. The tip of the drive cam is deformed, leading to an even earlier trip the next time. Also, this early trip can apply a damaging force to the close prop which latches the charged close spring. If the point of drop off of the cam profile is moved further around the drive cam, the tip at the drop off becomes vulnerable to damage, and/or the reset cavity is obstructed and the mechanism will not reset properly.
There is a need, therefore, for an improved electrical switching apparatus for power circuits which is not susceptible to premature trips.
This need and others are satisfied by the invention which is directed to electrical switching apparatus having an anti-rollover cam extendable beyond the tip of the close section on the drive cam cam profile to prevent a premature trip. This anti-rollover cam retracts to allow the drive roller to freely enter the reset cavity when unlatched by the latch mechanism.
More particularly, the invention is directed to electrical switching apparatus comprising at least one pole having separable contacts comprising fixed contacts and movable contacts and a carrier mounting the movable contacts for movement to open and close the separable contacts. An operating mechanism, which moves the contact carrier of each pole between the open and closed positions, comprises a closing spring assembly and a cam assembly coupled to and rotated by the close spring assembly. The cam assembly includes a drive cam mounted on the cam shaft and having a cam profile including a close section with a tip from which the cam profile falls off to form a reset cavity. The apparatus further includes a drive coupling connected to the carriers and a latch assembly selectively latching the drive coupling into engagement with the close section of the drive cam cam profile to drive the carriers to the closed position as the drive cam is rotated by the close spring assembly, and to unlatch the drive coupling allowing the drive coupling to drop into the reset cavity and move the carriers to the open position. The cam assembly includes an anti-rollover cam movable relative to the drive cam to an extended position extending the close section of the drive cam profile circumferentially beyond the tip to prevent rollover of the drive coupling into the reset cavity with the drive coupling latched, but retracting with the drive coupling unlatched and dropped into the reset cavity.
Preferably, the anti-rollover cam is pivotally mounted on the cam shaft and is biased by a bias spring to the extended position. In a particularly advantageous arrangement, the drive cam is formed by a pair of cam plates spaced apart on the cam shaft and the anti-rollover cam is mounted on the cam shaft between the pair of drive cam plates. Also preferably, the anti-rollover cam extends radially beyond the tip on the drive cam so that the forces are taken by the anti-rollover cam and the tip does not become deformed.
In another preferred arrangement, the drive coupling includes a drive roller which engages the drive cam cam profile and the latch assembly includes a reset spring which pulls the drive roller into the reset cavity. The reset spring is sufficiently stronger than the bias spring such that the anti-rollover cam is retracted by the reset spring pulling the drive roller into the reset cavity. The anti-rollover cam can have a transition surface, which is preferably arcuate between a radial facing surface and a circumferentially facing surface. This transition surface is configured to generate a circumferential component of force initiating retraction of the anti-rollover cam as the latch mechanism unlatches the drive coupling and the reset spring pulls the drive roller toward the reset cavity.